Hi there, we use have a very similar design in some of our pay-to-drive machines. Its actually very simple, but the one thing to make it work better is to try and make the robot as balanced as you can over the wheels. The closer you can get to making it balanced, the easier the lance will throw itself over the top.
The very end of the lance can be as sharp as you like, but i would recommend using something very smooth (a castor would break too easily) on the underside/topside of the lance to help with turning.